Definition:
Veto
from The SAGE Glossary of the Social and Behavioral Sciences

The power to block legislation. In the United States, the president can veto any bill presented to him by not signing it. He is required to return the bill to Congress with the proposed changes. Congress has the power to override the president's veto by a two-thirds majority of both Houses and have the bill become law.

See also

Bill, Law (political science)

Summary Article: veto

from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Exercise by a sovereign, branch of legislature, or other political power, of the right to prevent the enactment or operation of a law, or the taking of some course of action.

The term originates in the power of the tribunes of the plebs of ancient Rome to declare their protest against any unlawful measure.

The exercise of the veto has been of great importance in the operations of the Security Council of the United Nations. Each of the five permanent members of the Council, the USA, Russia, the UK, France, and China has the right to veto any resolution.

.BTXT:

In the UK the sovereign has a right to refuse assent to any measure passed by Parliament, but this has not been exercised since the 18th century. The power of veto enjoyed by the House of Lords was reduced by the 1911 Parliament Act to two years, and by an act of 1949 to one year. The House of Lords still has a suspensory veto on all legislation except finance measures, but this is seldom exercised

In the USA, the president may veto legislation, but this can be overruled by a two-thirds majority in Congress.

.UTXT:

Under the US Constitution, the president may veto legislation, although that veto may, in turn, be overruled by a two-thirds majority in Congress. Bills also become law if the president does not sign them within ten days of their passing through Congress, but this applies only when Congress is in session, so that bills which are presented for approval less than ten days before the end of the session may be subject to what is known as the president's pocket veto. This can assume considerable significance since there is a tendency for legislation to bank towards the end of a congressional session. A 1996 act gave future presidents a ‘line-item’ veto over specific items in spending bills, significantly increasing presidential authority. The act met with legal challenge, however, because it was seen as upsetting the balance of power between the executive and legislative branches of the government, and in 1998 the Supreme Court declared the line-item veto unconstitutional.